Energy-transducing thylakoid membranes remain highly impermeable to ions during protein translocation.
We investigated the operation of a posttranslational protein translocation pathway to determine whether ions are excluded from the translocase during protein transport. The membrane capacitance during protein translocation across chloroplast thylakoid membranes was monitored via electric-field-indicating carotenoid electrochromic bandshift measurements. Evidence is presented that shows that the membrane ion conductance is not increased during the complete cycle of binding, transport, and substrate release by the DeltapH-dependent translocase; i.e., the membrane remains ion-tight during protein translocation. We further demonstrate that a synthetic targeting peptide that directs proteins across this membrane does not gate translocation pores. We conclude that protein transport across the thylakoid membrane does not compromise its ability to maintain ion gradients and is, thus, unlikely to affect its functions in energy transduction.
Teter
S A
SA
Division of Biological Sciences, Section of Plant Biology, University of California, Davis, CA 95616, USA.
Theg
S M
SM
D001693:Biological Transport, Active; D002474:Cell-Free System; D002736:Chloroplasts / Q000378:metabolism*; D006096:Gramicidin / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D008564:Membrane Potentials; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011994:Recombinant Proteins
D045322:Photosynthetic Reaction Center Complex Proteins; D011994:Recombinant Proteins; D006096:Gramicidin